Background

In tropical coastal regions, sediment loads of rivers as well as remobilized alluvial deposits interact with marine processes. The associated reduction of water clarity restricts the diversity of coral and coral reefs, which may be considered the prime tropical marine ecosystems. The Bay of Banten, West Java, represents a region where corals survive under highly turbid conditions. Over ten coral reef islets can be found in the bay, located close to an abandoned river delta where erosion leads to retreat of the coastline (Fig. 1).

Fig. 1: Erosion of the inactive Ciujung delta system in Teluk Banten

Objectives

• To explain the occurrence of high levels of turbidity in the immediate vicinity of reef slopes
• To assess the dominant flow mechanisms inducing transport of fine-grained sediments towards reef localities

Methodology

Analysis of simultaneous observations on tidal and subtidal current velocities, seawater salinity, suspended sediment concentration and meteorological parameters. The islets Pamujan Besar and Kubur were selected to study local sedimentary processes close to reef fringes.

Results

Alongshore residual currents relate to the local wind field, implying an east- and westward throughflow during the Northwest end Southeast Monsoon, respectively. Wave-induced resuspension in the former delta region is largest during the Northwest Monsoon, when residual flows are eastward. Cross-shore circulation is limited, restricting offshore transport of sediments eroded at the shoreline (Fig. 2).

Fig. 2: Monsoon driven throughflow

Tidal currents remobilize sediments on the subtidal former delta plain. The asymmetry of tidal currents induces eastward sediment transport. A model has been set up to quantify the tide-induced sediment transport. (Fig. 3).

Fig. 3: Asymmetric tidal currents raise levels of suspended sediment concentration (SSC)

Clouds of sediments, centred at the reef toe, frequently surround the reefs (Fig. 4).

Fig. 4: Spatial pattern of SSC around a coral reef islet

Mean and maximum turbidity levels at the reef slopes increase with depth and generally surpass levels in the far field (Fig. 5).

Fig. 5: Depth dependence of mean (bars) and maximum (sticks) SSC

Spectra of suspended sediment concentration at reef slopes are concentrated in the diurnal frequency band (Fig. 6).

Fig. 6: Spectra of SSC at reef slopes

Current-induced shear stresses act on the entire reef slope (Fig. 7). Wave stirring is largest at the reef crest and decreases exponentially with depth. Stagnant waters prevail in the lee of the islets, where sediment accumulates. Eddies may occasionally occur. Scour trenches flanking the islets are the morphological response to current contraction.

Fig. 7: Three-dimensional current velocity pattern around a coral reef islet

Conclusions

Peak levels of turbidity in coral reef environments of the Bay of Banten are due to local hydrodynamic processes, raising clouds of sediments at the reef slopes. Dispersion of sediment suspensions towards the reef islets results from monsoon-driven circulations, tidal current asymmetry and residual sediment fluxes induced by spatial variation of current velocity amplitudes.

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